Throughout all the pages of
the present Seminar,
Microscopic World -1-,
we have learned that
matter and
light in the
microscopic world
possess both the
particle nature and the
wave nature.
In addition,
we could understand
that the Bohr model of
atomic structure
is constructed
by considering
both the natures.
By using this model,
it has become possible
to explain the stability
of atoms and
the atomic spectra
which was impossible
to be explained with the
classical theory
(Newtonian mechanics
and Maxwellian electromagnetism).
It seems that
the double nature
(the duality)
of matter and light,
i.e. the wave
and particle natures,
is a very fundamental
property of "things"
or "beings"
existing in our world.
In other word,
the duality seems
to be a fundamental philosophy
for the "existence"
of things.
This looks quite
strange and mysterious
for us,
because we are very much
used to the thinking way
in the classical theory.

[Young's Experiment]Let us look back
at Young's experiment
by which the wave nature
of light was confirmed
for the first time.
For the sake
of convenience,
the contents of
Young's experiment
presented in the page
3-8: Summary of Part 3are shown here again.

Schematic drawing of Young's experimentThe monochromatic light
from the source passes
through the two slits,
S1and
S2,
and produce an interference
pattern (fringes)
on the screen.

An example of
the picture of the pattern
is represented in
the following picture (A)and (B).
In these pictures,
(A) shows the result when
one of the slits is closed,
and (B) the result when
both of the slits are opened.

We will discuss
on some details
of these pictures below.
Before that,
let us study
a little about
the relation between the basic
mechanism of
photographyand the particle nature
of light.

[The Mechanism of Photography
and the Light Particle]In Young's experiment,
the reason that
the photographic plate
(or film)
can response
to light is due to
the particle nature of light.
Let us explain this below.
Photography is based
on the chemical reactions
brought about
by the action of light
(photochemical reactions).
Namely, the molecules
of halogenated silver
are dissociated
by the illumination
of light
and silver atoms
are educed.
For example,
silver bromide
(AgBr) that is
one of the halogenated silvers
is dissociated by light as
AgBr
-->
Ag + Br
to be silver
and bromine atoms.
After being treated
with special chemicals,
only the silver atoms
remain to be black coloured
and to make
a photographic negative.
To dissociate a molecule
of halogenated silver,
roughly speaking,
more energy than
about 2 eV
is necessary.
Namely, one molecule
of silver bromide
(AgBr)
or silver iodide
(AgI)
can be dissociated
only by the accumulation
of more energy than
about 2 eV
This amount of energy
is almost the same as
the value of the
work functionwhich is the
necessary minimum energy
to bring about
the photoelectric effect.
(Refer to the page
3-6: The Hypothesis of Light Quanta and the Photoelectric Effect.)
Hence, the discussion
on that page
about the time
to make the photoelectric effect
take place
is valid for the present
photochemical reaction
as well;
i.e., if we consider
the time necessary
for the dissociation
of the halogenated silver
within the classical theory,
it should take
a time longer
than the shutter speed
of an ordinary camera
that is usually shorter
than 1/100 s or 1/1000 s.
Consequently,
we cannot help
considering that
light can dissociate
the halogenated
silver molecules
being absorbed instantly
as a particle of light(photon).

[The Mysterious Particle-Wave Duality]The image of
the interference fringes
recorded on the
photographic plate
(film)
is consists of
huge number of silver atoms
educed by the photochemical
dissociation of
halogenated silver molecules.
These silver atoms collect
together to make the
interference pattern
on the film.
We recognize this collection
of the silver atoms
as a picture of the
interference pattern.
Namely, we are looking at
the collection of
marks or
traces of photons.
Each mark denotes
a point where
a particle of light
(photon) has struck.
The collection
of these huge number
of marks
constitutes a interference
fringes which characterize
the wave nature of light.
How can we understand
this mechanism?
If light were
simply particles,
one light particle
could not pass through
two slits
simultaneously,
so that it could never
produce interference fringes.
If light were simple
particles,
then the picture
on the screen
should be what
is obtained by superposing
such a picture
in the case of
a single slit opened
as shown in
the above picture (A)with another picture
obtained by displacing it
by the interslit
distance d.
However, we have
an interference pattern
as shown in
the picture (B)in practice.
Hence, we must consider
that light is not
a simple particle,
but possesses some kind
of wave nature as well.
If light were
pure waves,
we could not have
a picture of
the interference fringes.
Because light has
the dualityof wave nature
and particle nature
simultaneously,
we can have the picture
of the interference pattern.
We therefore cannot
deny the dual
nature of light.
Then, what part of light
is a wave?
And what part
of light is a particle?
You might suspect that
two different photons
interfere with each other
after they pass through
two slits separately
and consequently
they produce the
interference pattern
on the screen.
However this idea
is not valid
by the following reason:
If the intensity
of the incident light
to the double-slit
experiment is so weak
that only one photon
runs at every
moment of time,
two different photons
are impossible
to pass through two slits
at one time
and
no interference occurs
among different photons.
In spite of such extreme
weakness of the intensity
of the incident light,
the same interference pattern
is still obtained
on the photographic plate
by exposing it for
an extremely long time.
Such an experiment of
three-month exposure time
was carried out
in 1909 by a British student,
and he got a clear
interference pattern
that is exactly the same as
those in usual
Young's experiment.
This tells us that
the interference pattern
in Young's experiment
is not caused
by the interference
between different photons.
Then, why does the
interference pattern
come into existence?

[Opening to Quantum Mechanics]As mentioned above,
we have encountered
a serious discrepancy
between
the particle natureand the wave natureof light.
This discrepancy
is not only
for light but also
for electrons.
In order to perfectly
overcome this discrepancy,
people have had
to construct a completely
new theory
getting over the
difficulties in the
classical theory.
This is nothing else
than Quantum Mechanicsproposed by
W. K. Heisenberg(Germany, 1901 - 76)
and
E. Schroedinger(Austria, 1887 - 1961).
By this quantum mechanics,
the mystery of the particle
and wave natures
of light and electrons
were solved and
the atomic structure
was clarified.
You are strongly expected
to challenge to study
Quantum Mechanicsproceeding to
the Second Part of
the present seminar,
Microscopic World -2- (Introduction to Quantum Mechanics).